Resistance Temperature Detectors (RTDs) are able to measure temperature using the natural correlation temperature has with the electrical resistivity of metals. RTDs are great devices to use for measuring the temperature of liquids and gases because of their long-term stability, ease of recalibration, repeatability, and precision over relatively narrow temperature spans.
Traditionally, Resistance Temperature Detectors (RTDs) were wire-wound devices that included a length of wire wrapped around a probe. To protect this wire-wound RTD from the external environment, a metal sheath or cover was placed over the device. Adding the cover, however, reduced the RTD's response time as heat had to first transfer through the cover before reaching the RTD.
Today, RTDs are commonly made by disposing a conductive metal film onto a semiconductor substrate and etching the conductive metal film into a resistance temperature detector pattern. Relatively cheap metals such as copper or nickel are sometimes used as the conductive metal. However, these metals are restricted in their temperature measurement range because of their non-linear temperature-resistance relationships. To avoid this set-back, most RTD designs use platinum as the conductive metal. Platinum is desirable because its chemical inertness and nearly linear temperature-resistance relationship enable an RTD to precisely measure temperature. To remain stable, however, platinum must remain in its pure form and be shielded from high temperatures and harsh environments. A variation of covers have been used in the past to shield platinum from external environments, however these covers share the drawbacks mentioned above of having a slow response time due to heat being transferred slowly through the cover prior to being measured by the RTD. Because of the slow response time of such RTDs, they are not commonly used in applications requiring rapid response. This is especially true in high temperatures and harsh environments, such as those commonly found in automobile engines and jet engines. Accordingly, there is a need for a fast response RTD that operates in high temperatures and harsh environments.